1. Field of the Invention
The present invention relates, generally to a method for time synchronization of a mobile station in a radio communications system and more specifically to a mobile station and to a base station which can carry out such method.
2. Description of the Prior Art
The design of digital radio communications systems is shown in J. Oudelaar, xe2x80x9cEvolution towards UMTSxe2x80x9d, PIMRC 94, 5th IEEE International Symp. on Personal, Indoor and Mobile Radio Communications, The Hague, NL, Sep. 18-22 1994, pages 852-856, and M. Lenti, H. Hageman, xe2x80x9cPaging in UMTSxe2x80x9d, RACE Mobile Telecommunications Workshop, Vol. 1, Amsterdam, NL, May 17-19 1994, pages 405-410.
The presently known mobile radio system GSM (Global System for Mobile Communications) is a radio communications system with a TDMA component for subscriber separation (time division multiple access). User information from the subscriber connections is transmitted in time slots in accordance with a frame structure. The transmission is performed in blocks. Furthermore, frequency channels (SCH synchronization channels) matched to the timing pattern of the frame structure and serving the purpose of time synchronization for the mobile stations are known in the downward direction from the GSM mobile radio systems. In this frequency channel, a mobile station can evaluate transmitted data for the purpose of self-synchronization. Such self-synchronization is disclosed in EP 0 551 803 A1.
A method for time synchronization for a mobile station is known from EP 0 767 557 A1. The connections are separated in a pure TDMA system by time slots and frequency bands. The base station transmits a frequency channel for, time synchronization (SCH) in a fixed fashion at regular intervals. This frequency channel for time synchronization is always arranged at the same position in a multiframe; for example, in each case in the first time slot of each tenth TDMA frame. A 64-bit training sequence contained in the frequency channel for time synchronization, and the location of its arrangement within the TDMA frames and time slot, are known to the mobile station.
DE 195 49 148.3 discloses a mobile communications system which uses a TDMA/CDMA subscriber separation (CDMA code division multiple access), and applies at the receiving end a JD method (joint detection) in order to undertake with knowledge of spread codes of a plurality of subscribers, improved detection of the transmitted user information. Information from a plurality of user data connections which can be distinguished by their spread code are transmitted simultaneously in one frequency channel (TCH traffic channel). However, the division of specific frequency channels for synchronization purposes results in a large loss in capacity in comparison with the GSM system since the frequency range used for a frequency channel is a more broadband one.
It is an object of the present invention, therefore, to provide a method and apparatus which permit time synchronization in conjunction with low consumption of radio resources in a TDMA radio communications system.
A radio communications system, for example a TDMA/CDMA radio communications system, provides frequency channels which are formed by time slots and by broadband frequency ranges, and in which information from one or more connections is transmitted simultaneously between mobile stations and base stations; it being possible to distinguish the information from different connections in accordance with a connection-specific fine structure. However, it is also possible to allocate to a connection a plurality of different fine structures which are recombined at the receiving end.
According to the present invention, frequency channels in which a known signal form is transmitted, in addition to information from further connections, are temporarily repeatedly provided for time synchronization for the mobile stations in the downward direction. The known signal form is transmitted by a first base station, while the further connections are supplied by other base stations. As an alternative to this, it is also possible for an individual base station to transmit the known signal form for synchronization, as well as information from the further connections, in one frequency channel. At least one instant of the arrival of the signal form is determined by the mobile station to be synchronized from received signals, which is subsequently used by the mobile station for time synchronization.
The radio resources of the air interface between the base station and mobile stations can be better used by virtue of the fact that the signal form is transmitted in addition to information from further connections. Because information can be distinguished in accordance with an impressed fine structure, a time slot is not blocked solely by the synchronization, rather, it can be used multifariously. The further connections can, in this case, be user data connections or signaling connections. In a simultaneous network, that is to say adjacent base stations that use the same frequency range, the further connections are offered, for example, by the adjacent base station.
In accordance with an embodiment of the present invention, the signal form is designed as a training sequence of a message block with data to be transmitted. Thus, message blocks which transmit information for various purposes can be used by configuring a specific training sequence for time synchronization. The capacity of the air interface is, thereby/ further increased. The training sequence can be used further for channel assessment.
The signal form is advantageously spread with the aid of an individual spread code, wherein it is possible for the information on the presence of a signal form to be included in the spread code for time synchronization. The signal form also can be interpreted as a sequence of chips which generate a bandwidth which fills up the frequency range. An additional outlay on processing at the receiver is eliminated by virtue of the fact that the message block with the data for time synchronization can be processed together with the remaining message blocks by despreading.
In accordance with an embodiment of the present invention, the at least one instant is determined by determining the correlation between a received signal and a reference sequence known in the receiver. A correlation peak determined in this case specifies, f or example, the temporal mean of a time slot. Thus, it can serve the purpose of further orientation with reference to the frame structure of the air interface. Alternatively, the at least one instant can be determined by signal-matched filtering of a received signal.
In order to increase the accuracy of time synchronization, the instant of the arrival of the signal form is advantageously determined several times and averaging of the instants is carried out for the purpose of frequency synchronization. The averaging reduces the influence of single errors.
For frame synchronization of the mobile station, the signal form, which is expressed, for example, as a digital symbol sequence, includes data on a frame structure of the frequency channels in specific time slots. The signal form, therefore, also can be used to transmit this information without further loss of capacity. The symbol sequence at specific instants of hyperframes and/or superframes advantageously differs from the symbol sequence in the remaining time slots of the frequency channel for time synchronization. As an alternative, however, it is also possible for the purpose of frame identification for additional symbols to supplement the symbol sequence at specific instants; for example, at the start of hyperframes and/or superframes.
Additional features and advantages of the present invention are described in, and will be apparent from, the Detailed Description of the Preferred Embodiments and the Drawings.